At NASA's Johnson Space Center, there is a microgravity research
aircraft, a KC-135 Low-G Flight Research aircraft (a predecessor
of the Boeing 707) which is typically used for refueling military
aircraft. The four-engine turbojet has been modified to meet NASA's
needs to train astronaut and conduct microgravity research. The
biggest difference is the way that it is used to fly parabolas
to create 20-25 seconds of weightlessness so that the astronauts
can experience and researchers can investigate the effects of
"zero" gravity. A typical mission is 2 to 3 hours long
and consists of 30 to 40 parabolas. These parabolas can be flown
in succession or with short breaks between maneuvers to reconfigure
test equipment. The gut wrenching effects have earned this aircraft
its famous nickname: "The Vomit Comet."

As NASA's lead center for microgravity research in the areas of
fluid physics and combustion science, Glenn hosts the KC-135 for
6-10 weeks each year in support of its ground-based microgravity
research. Some flights include astronauts participating in crew
training so that they can have low-g experience with experiment
hardware prior to shuttle missions.